FIG. 1 illustrates a cross-section of an example solder ball connection between a package board and a module board according to the prior art. A package board carries one or more chips (e.g., memory chip(s), logic chip(s), analog/RF chip(s), and/or etc.). A module board serves a board for connection to a mother board such as in a computer, or may serve as the main device board such as in a cellular phone.
As shown in FIG. 1, the conductive pad 8 of a package board 10 is formed on laminate 6 of the package board 10. The conductive pad 8 of the package board 10 includes a copper layer 12 having a layer of nickel 14 formed thereon. A gold layer 16 is formed over the nickel layer 14. Photo solder resist (PSR) 18 is formed adjacent to the nickel and gold layers 14 and 16 on the copper layer 12. The conductive pad 22 of the module board 20 may have the same structure as the package board 10 and may also include a gold layer 16. A solder ball 24 makes electrical contact with the conductive pad 8 of the package board 10 and the conductive pad 22 of the module board 20.
FIG. 2 illustrates another example of the solder ball connection between a package board and a module board according to the prior art. As shown in FIG. 2, the conductive pad 28 of the package board 30 includes a copper layer 32 formed on a laminate 26. A photo solder resist (PSR) 34 is formed over and exposes a portion of the copper layer 32.
The exposed portion of the copper layer 32 is subjected to the well-known organic solder-ability preservative (OSP) treatment. The resulting OSP coating (not shown) has strong adhesion to copper and strong resistance to oxidation. As such the OSP coating, usually 0.2 to 0.4 um thick, helps prevent the exposed portion of the copper layer from oxidizing. When the conductive pad 28 is to be soldered, the OSP coating is removed. The conductive pad 38 of the module board 40 may have the same structure as the package board 30. A solder ball 42 makes physical and electrical contact with the copper layer 32 of the package board 30 and the module board 40.
In the past, solder balls such as in the above-described embodiments included lead. However, in the interest of protecting the environment, a lead free alternative has been developed. For example, alloys of tin, silver and copper have been developed as lead free alternatives. Unfortunately, lead free solder has proven to be more brittle than leaded solder. Consequently, device failure due to, for example, cracking of the solder ball joints is of greater concern.